The prior art has recognized, as described in the article entitled "The Use of R--R Interval and Difference Histograms in Classifying Disorders of Sinus Rhythm" by P. M. M. Cashman appearing in the January 1977 issue of Journal of Medical Engineering Technology, the need to provide heart activity and in particular ECG recording over relatively long period of time from ambulatory patients. The primary objective of such recordings is to permit identification of infrequent and transient disturbances of cardiac rhythm, which may be important in diagnosing patients with vague or inermittent symptoms such as dizzy spells, blackouts, and fainting attacks. While recording for a longer period of time, the physician's interest is to detect short, specific dysrhythmic events which occupy only a small percentage of the total recording time. Such disrhythmic events are considered as singularities in a background rhythm. Typically, the physician is interested not only in the specific disrhythmic event but also the background rhythm which may comprise slower responses of the heart to influences such as drug treatment or psychological stress over long periods of time. In this regard, it may be desirable to compare the long recordings from either different patients or from the same patient at different times.
At present, the use of ambulatory heart monitors, typified by the Holter recorder, is well known in the art. The Holter technique typically records the patient's ECG activity for at least 24 hours. The difficulty with the use of a Holter recorder is that it provided a large volume of information which requires processing and analysis. Such analysis is usually time consuming and expensive. Typically, the review and processing time may be reduced by reading out and displaying the data at increased speeds, typically multiples of 25, 32, 60, and 120 of the normal playback rate. In order to display the heart data, and in particular the QRS complex, with sufficient clarity, it may be necessary to use recording apparati with fequency responses in excess of 12 kHz. This is possible with ultraviolet recorders, fiber optical recording oscilloscopes, or inklet records, all of which are expensive.
The prior art has suggested a variety of apparati for the processing and display of a patient's heart activity data. Such apparatus may include a detector for automatically sensing a dysrhythmia and in response thereto, stores and displays a sample of the patient's ECG either directly onto paper or onto a screen. Further, a contourogram may be provided by a storage oscilloscope or Polaroid camera whereby subsequent lengths of a patient's ECG are displayed one beneath the other. Each such segment or tracelength is proportional to the beat-to-beat interval and the right-hand edge of the trace gives a continuous record of the R--R interval trend. One of the most common methods of presenting ECG data is to provide a heart rate trend wherein the R--R interval is continuously measured and the R--R interval data is segmented into continuous time periods. In particular, the rate or interval of the heartbeat is averaged ot provide a display of average heart rates or intervals for a series of adjacent time periods. For resolving individual beats on a trend plot, it may be preferable to plot the instantaneous rate in a form of a trend plot.
In addition to the methods of display as discussed above, there is the need to identify and to highlight the isolated occurrence of an abnormal or ectopic beat. Typically, the ectopic beat is detected and is counted during the course of the heart monitoring period. Apparatus has been provided to detect premature heartbeat. The degree of prematurity can often be preset by the operator to provide selection of the beats to be detected. The output of such detectors is applied to a counting circuit, a trend recorder, or some form of alarm depending upon the desired application. Several techniques have been used to detect the abnormal shape of the patient's ECG signal including simple integrators, filters, and digital pattern recognition systems. It is contemplated that normal ECG signals are analyzed to determine the limits of occurrence of the peaks of the QRS complexes, and to compare such limits with the incoming signal to determine whether the present ECG signal fits within these predetermined, normal limits.
In more sophisticated systems, the patient's normal ECG signal is detected and stored. In subsequent monitoring, the patient's normal signal is used as a template against which to compare the current heart data. U.S. Pat. No. 4,115,864 discloses such a cardiac monitor, utilizing a computer to control the storage of the patient's normal signal, to process the inputted ECG signal into digital segments, and to compare those segments with corresponding segments of the previously stored normal ECG of the patient. When the stored normal signal does not fit the current signal, an alarm or detector circuit is actuated and the number of occurrences of that ectopic signal is stored within the memory of the computer. The described system is capable of providing a trend history of such ectopic beats in the form of a histogram or trend plot.
In any processing of ECG data, it is necessary to use discrimination techniques to extract a trigger pulse for each cardiac cycle. In most instances, the detection circuit triggers on the R-wave, and it is thus necessary to distinguish the R-wave of the QRS complex from the rest of the signal. It is contemplated that it may be necessary, in the presence of heavy background noise, to terminate the monitoring for the duration of the high level background noise. In other instances, the detection circuitry may provide an alarm so that the operator can intervene.
Noting the desire to provide monitoring for long periods of time of a patient's heart activity, the continuous monitoring by the Holter monitor provides a complete record at relatively inexpensive cost. On the other hand, the methods of processing and compacting the data for simpler, limited display oftentimes requires a more complex system of increased cost. To avoid either pitfall, it has been suggested that the data be presented in the form of an R--R interval histogram as shown in FIG. 1. A histogram presents heart activity data taken over a period of time in a compact manner, wherein the successive intervals between R-waves are computed and are classified as to their duration. As shown in FIG. 1, the X axis is in seconds corresponding to the R--R interval, whereas the Y axis provides the number of beats that occur within each interval. Such a presentation provides a large data reduction in a visual way which allows easy comparison between the histograms of the same patient taken at different times and between different patients. The R--R interval histogram (IH) is formed by generating an array of columns or bins, each corresponding to a range of values of beat-to-beat (or R--R) interval. As each ECG complex is detected, the time interval between it and its predecessor is measured and the total in the appropriate bin incremented. A typical histogram might contain a hundred bins, each having a width of 20 milliseconds, giving a total range of R--R intervals from 0 to 2 seconds. A bin capcity of 4,095 beats (a 12-bit binary word) will permit about 4 hours of normal heart monitoring.
A variation of the R--R interval histogram is the R--R interval difference histogram (IDH), as shown in FIG. 2. The IDH is formed similarly except that the quantity as displayed along the X axis as shown in FIG. 2 is the amount by which the R--R interval changes between successive beats. As shown in FIG. 2, the central bin is designated 0, i.e., 0 difference between successive beats, and a hundred columns provide a range between -1 second through 0 to +1 second, with a bin width of 20 milliseconds. The mean of the interval differences is always very close to 0. The IDH provides an indication of the manner of change of the heartbeat, where the width of the IH is the measure of the spread of heart rates about the mean value. It is apparent that the use of both the IH and the IDH provide the physician with a powerful tool for diagnosis of the patient's heart.
Further, U.S. Pat. No. 4,146,029 describes a system implanted within the body of the patient for dispensing medication into the patient's body. The system is implemented by a microprocessor for the control of the process whereby each QRS complex of the patient's heart is detected as to the length of the QRS complex and to the interval therebetween. A program of comparing the length of the QRS complex to an acceptable length is provided to determine the validity of each QRS complex and further to first calculate the interval between QRS complexes and to compare the measured length to a known or standard length for a particular patient. More specifically, the process compares the length to determine how much shorter the measured ongoing intervals are with regard to the normal length and dependent upon the decrease the length, i.e., the increase of heart rate, the microprocessor controls dispensing apparatus to vary the dosage given to the patient. Included in the appartus is a process for measuring the R--R interval and for accumulating the average of the R--R interval over a given period of time, for example, an hour. The averaged R--R intervals are compared with known standards to variably control the dispensation of medication to the patient.